This invention relates to photodetector circuits. More particularly, this invention relates to the portions of a photodetector circuit that are used to convert a light signal to an electrical signal.
A photodetector is a device used for converting the photon energy of a light source into a photocurrent. In general, photodetectors are used in many practical applications such as communications, security systems, optical analyzing instruments, and other modern electronic devices. Typically, photodetector circuits include an amplifying stage connected to the photodetector for producing an analog voltage signal proportional to the photocurrent.
In many applications, the analog voltage signal from the amplifying stage is applied to a comparator circuit in order to determine the relative strength of the light signal with respect to a preset threshold voltage. If the analog signal is less than the threshold voltage, the comparator circuit may trip and produce a logic high at its output. However, if the analog signal is greater than the threshold voltage, the comparator circuit may generate a logic low at its output.
This type of circuit is often used to create an object detector that can determine the presence or absence of an object within a specific region. For example, a photodetector circuit may be arranged so that a light emitting diode (LED) constantly projects a beam of light on to the photodetector. As long as the light beam is uninterrupted, the photodetector produces a sufficient photocurrent, and the output of the comparator circuit remains at a logic low. However, when an object blocks the light signal, the photodetector turns OFF, and the output of the comparator circuit becomes a logic high.
A block diagram of a typical prior art photodetector system 100 is shown in FIG. 1. System 100 generally includes a photodiode 110, an amplifier circuit 120 and a comparator circuit 130. Infrared light incident upon photodiode 110 creates a small photocurrent that is changed into a voltage and amplified by amplifier circuit 120. Comparator circuit 130 compares the amplified signal with a preset threshold value to determine whether it is greater or less than the threshold value. If the amplified signal is less than the threshold, comparator 130 trips generating a logic high at its output. This means that the level of infrared light sensed by photodiode 110 has fallen below a minimum value, indicating a lack of infrared light. When the amplified signal is greater than the threshold value, comparator 130 supplies a logic low at its output, indicating that a sufficient amount of infrared light is detected.
Photodetector system 100 may be used in conjunction with other components for determining the presence or absence of objects. For example, assume a light emitting diode (shown in FIG. 2 as LED 160) is positioned to constantly project a beam of infrared light upon photodiode 110. This creates a xe2x80x9csensing fieldxe2x80x9d 170 between LED 160 and photodiode 110 within which objects can be detected. As long as the path of light from LED 160 to photodiode 110 is uninterrupted, a sufficient amount of current will be continuously generated by photodiode 110 so that the output of comparator 130 remains at a logic low. However, when an object enters sensing field 170, the beam of infrared light is interrupted and photodiode 110 turns xe2x80x9cOFFxe2x80x9d (i.e., ceases to conduct). This causes comparator 130 to trip and produce a logic high at its output. The presence or absence of an object within sensing field 170 may be determined by examining the output of comparator 130. If the output is a logic high, an object is present in the sensing field, if the output is a logic low an object is not present.
A schematic diagram of photodetection system 100 is shown in FIG. 2. As shown in FIG. 2, photodiode 110 is reverse-biased with its anode connected to ground through resistor 111 and its cathode connected to a +5V power source. LED 160 is forward biased with its anode connected to +24V power source and its cathode selectively connected to ground through one of resistors 151-158. Each resistor has a different value (151 greater than 152 greater than  . . . 158). The intensity of the infrared light supplied by LED 160 is determined by the value of the resistor it is connected to. For example, if LED 160 is connected to the resistor with the smallest value (i.e., resistor 158), a relatively large amount of current will flow through it, causing LED 160 to produce the greatest quantity of light. Conversely, If LED 160 is connected to the resistor with the largest value (i.e., resistor 151), a relatively small amount of current will flow through it, causing LED 160 to provide the least quantity of light.
When photodiode 110 receives an infrared beam from LED 160, the reverse-bias leakage current across it increases causing a voltage to be generated across resistor 111 (RL). This voltage is applied to the non-inverting input of operational amplifier (op-amp) 120 through a high-pass filter 112 formed by capacitor 114 and resistor 115. The gain of op-amp 120 is controlled by the ratio of resistors 116 and 117. The amplified output voltage Vo of op-amp 120 is applied to the inverting terminal of comparator 130. A threshold voltage VR created by series coupled resistors 131 and 132 is applied to the non-inverting terminal of comparator 130. When the output voltage of op-amp 120 is less than the threshold voltage, the output of comparator 130 is high, when the output of op-amp 120 is less, it is low.
The output signal of comparator 130 is fed to a microcomputer 140. Microcomputer 140 reads this output signal so that it may determine whether an object is present within sensing field 170. In addition, the output of op-amp 120 is also fed to microcomputer 140 so that it may monitor the magnitude of the amplifier""s output signal. Microcomputer 140 selects a resistor value based on this output signal and transmits a control signal to driver 150 in order to adjust the intensity of the light produced by LED 160.
One deficiency of photodetection system 100 is that LED 160, which is connected to the +24V power supply, is constantly on and therefore consuming a significant amount of power. In addition, changing the overall gain of photodetection system 100 poses several practical problems. For example, changing the gain of system 100 involves adjusting the value of the feedback resistor connected to an input pin of op-amp 120. This is generally an undesirable way of adjusting the gain due to the sensitivity of the op-amp to conditions at the input pin. Furthermore, having an external pin that connects to the input of op-amp 120 adds parasitic capacitance to the op-amp""s input, which reduces its phase margin. Although this problem can be solved by increasing power consumption, this solution is undesirable.
Thus, in view of the foregoing, it would be desirable to provide a photodetector circuit that operates at reduced power levels. It would also be desirable to provide a photodetector circuit that has an overall gain that can be adjusted by connecting an external resistor to a single external package pin.
It is therefore an object of the present invention to provide a photodetector circuit that operates at reduced power levels.
It is another object of the present invention to provide a photodetector circuit that has an overall gain that can be adjusted by connecting an external resistor to a single external pin.
In accordance with these and other objects of the present invention a photodetector circuit suitable for acquiring and reporting data indicative of the relative strength of a light signal is provided. The photodetector circuit is configured to alternate between standby and active periods in order to reduce overall power consumption.
The photodetector circuit includes a photodiode and a switch timing circuit that periodically generates a control signal. When the control signal is a logic low, an active period is initiated during which the photodetector may acquire and report light signal information. During a portion of the active period, an external LED is activated, causing it to generate a light signal. Assuming a sufficient amount of that light signal is reflected onto the photodiode, it conducts and generates a photocurrent. A variable gain transimpedance amplifier coupled to the photodiode amplifies the photocurrent and produces an analog voltage proportional to the amplified photocurrent.
While in the active mode, an auto-zero amplifier circuit coupled to the transimpedance amplifier amplifies the analog voltage and applies it to a comparator circuit. The comparator circuit compares the amplified analog voltage with a preset threshold value to determine whether it is greater or less than the threshold value. If the amplified signal is less than the threshold, the comparator trips generating a logic high at its output. If the amplified analog voltage is greater than the threshold value, the comparator supplies a logic low at its output. A digital filter may be coupled to the output of the comparator circuit to filter out spurious readings.
However, when the switch timing circuit generates a logic high control signal, the LED turns OFF, and the photodetector enters a standby mode. During the standby mode, the auto-zero amplifier initializes itself (i.e., sets its output to substantially zero), and the output of the comparator is set to a default logic state (i.e., either a logic low or logic high). Both the auto-zero amplifier the comparator remain in these respective disabled states during the standby mode so that no data is acquired or processed. In this mode, the power consumed by the photodetector circuit is significantly reduced. The photodetector circuit does not re-enter the active mode until switch timing circuit generates another logic low control signal.
These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings.